Changes in ozone (O3) as a result of human activity have important implications in the chemical evolution of the atmosphere, climate change and air quality. However, there is limited tropospheric O3 study in tropical and subtropical Asia and especially in China despite the fact that there is an unprecedented urban, industrial and agricultural development in the region. This thesis intends to study the characteristics of vertical O3 distribution and O3 transport in the troposphere over Hong Kong with an aim to identify the major sources of O3 in the subtropical atmosphere. The O3 distributions are studied through a total of 172 ozonesonde soundings and 3-year O3 and carbon monoxide (CO) measurements from a background monitoring station in Cape D' Aguilar, Hok Tsui. The O3 and precursor transport is studied through isentropic back air trajectory and meteorological analysis. Satellite images and chemical tracer measurements from the MAPS experiment and the PEM-West B mission are used to trace the sources of transported and in-situ produced O3. The effects of O3 change on climate in South China region are assessed through empirical formulation and field data. Tropospheric O3 shows a strong seasonal cycle with a maximum in spring and a minimum in summer. Ozone in the free troposphere has a characteristic board springtime peak and a summertime minimum extending from the boundary layer to the mid troposphere. It also has periodic low value in the upper troposphere in autumn and winter. Surface O3 shows a unique seasonal distribution with a major peak in autumn and a trough in summer. The winter and spring seasons are transition periods with a minor peak in spring. The biomass burning emissions associated with the active fires in Southeast Asia (SE Asia) continent are found to cause substantial O3 enhancements in the lower troposphere in springtime over Hong Kong. Satellite images and chemical signatures confirm that the biomass plumes associated with large-scale fires in SE Asia are transported to downwind South China and the western Pacific. Evidences show that the observed springtime O3 peak in the lower troposphere and some portions in the upper troposphere over Hong Kong is result of photochemical O3 production from the SE Asian biomass burning emissions. The finding is different from that in the mid latitudes of the Northern Hemisphere, where springtime O3 maximum is attributed to the active O3 intrusion from the stratosphere and/or photochemical O3 build-up from urban and industrial emissions. Widespread tropospheric O3 enhancement was observed over Hong Kong and most tropical and subtropical western Pacific in 1997 when huge amount of biomass was burned in the forest fire in Indonesia. The fires and O3 build-up were directly related to the unexceptionally drought conditions and abnormal tropospheric environment associated with the strong El Nino phenomenon. This phenomenon caused the descending arm of the Walker Circulation to shift towards the tropical Western Pacific and resulted in an abnormal high-pressure anticyclone over the region. The subsidence associated with the anticyclone caused photochemical O3 produced from biomass emissions to accumulate in the troposphere and outflow to neighboring regions. Calculations show that the O3 enhancements induced a maximum increase of 0.48 W/m2 in radiative forcing and 0.26C in surface temperature. Low O3 concentration was commonly observed in the upper troposphere during autumn and winter over Hong Kong. Aircraft measurement reveals that such low O3 is large-scale in nature and is also found in the upper troposphere of the whole of tropical and subtropical western Pacific. Meteorological analysis and chemical signatures show that the low O3 is a result of active convective transport of O3-depleted marine air to the upper troposphere in the tropics from where it is carried to the sub-tropical region following the meridional circulation of the East Asia local Hadley Circulation. In the surface layer, background O3 and CO show a strong and non-linear relationship throughout spring, summer and autumn. The import of O3 in the prevalent air mass accompanying the long-range transport process associated with the East Asian monsoon and climate system is the dominant factor governing its fluctuation in South China. In particular, outflows of pollution from the Asian continent and East Asian coast and the photochemical O3 produced from the Asian emissions are identified to be important sources of O3 downwind in South China and the western Pacific.